The long-term goal of this project is to determine how viral glycoproteins direct membrane fusion. Membrane fusion is a fundamental step in the life cycle of enveloped viruses and is responsible for the penetration of the host cell. Some types of viruses, notably paramyxoviruses such as Newcastle disease virus (NDV), can fuse at neutral pH with the plasma membrane of the host cell. These viruses also have the property of inducing syncytia formation, an important cytopathic effect as well as an alternate means of virus spread through a tissue. NDV membrane fusion requires the presence, in the same membrane, of the two viral glycoproteins, the HN protein and the F protein. Current evidence suggests the hypothesis that the HN protein serves to activate the activity of the F protein which is directly responsible for the fusion event. Furthermore, current evidence suggests a sequence of events leading to the formation of syncytia, and experiments to test the predictions of this model form the basis of this proposal. The project will make use of an extensive collection of mutants in both the HN and F protein genes. Specific Aims: 1. To define how the fusion protein directs the fusion between the attack and the target membranes. 2. To test the hypothesis that the HN protein functions to activate the fusion protein and to define the mechanisms involved in activation.